We are developing and applying new statistical methodologies to research in epidemiology. This year, work has progressed in 4 areas: (1) we have continued work on the problem of how to analyze data that include correlated outcomes, such as arise in pair-matched studies or in reproductive histories, when history of spontaneous abortion is to be linked to possibly embryo-toxic exposures; (2) we have completed work on the problem of imputing data for gaps that occur when an exposure history must be reconstructed retrospectively and we have begun work on the related and ubiquitous problem of how to handle study subjects with missing data in epidemiologic studies; (3) we have extended our models for fertility to allow for exposures that may change from day to day and may have toxic effects specifically on sperm; and (4) we have described and demonstrated a method to identify excess co-incidence of distinct birth defects, evidence for shared and transient - hence potentially environmental - etiologic factors, and we have now applied this method to demonstrate an etiologic association between sex organ defects and club foot in Norway.